There has heretofore been provided floor mounted rotatable jib cranes in which a vertical mast is supported for rotation within a pipe fastened to the floor or foundation. A jib or boom is secured to the upper end of the mast projecting horizontally and swinging in a horizontal arc as the mast is rotated.
A traveling hoist is supported by the horizontal boom. The hoist is mounted so as to be able to traverse along the length of the boom.
In such apparatus, it is critical that the boom extend in a true horizontal plane inasmuch as any out of level condition would possibly allow the traversing hoist to roll out of position due to a horizontal component of the load thus acting to urge the hoist and load to advance in either direction on the boom.
Also, the weight of an out of level boom itself could also cause self-induced rotation of the mast. U.S. Pat. No. 4,688,688 issued on Aug. 25, 1986 for a "Jib Crane Arrangement Having a Rotatable Mast" describes such a rotatable floor mounted jib crane in which the mast is rotatably mounted within a floor anchored pipe.
U.S. Pat. No. 4,511,048 issued on Apr. 16, 1985 for a "Jib Crane System Having a Rotatable Mast" describes an adjustable bearing arrangement for such apparatus for the purpose of maintaining and adjusting the plumb condition of the mast. This arrangement does not allow compensation for varying hoist loading conditions which can cause the boom to be deflected out of level or for other conditions which cause the boom level to be affected without any loss of out-of-plumb condition of the mast.
In addition, the bearings are relatively difficult to adjust, inasmuch as the bearing loading is effected by these adjustments limiting the extent of adjustment available for setting a plumb condition.
The above-described prior art jib crane arrangements have provided a 360.degree. rotation capability for booms affixed to the mast. For very long length booms, a cable and turnbuckle are attached to the outer end of the boom and the upper end of the mast in order to resist load-induced sagging of the boom. In such arrangements, the pivoting heretofore provided has been comprised of pivots of the boom to the mast and the anchoring point of the turnbuckle cable combination.
Such arrangement does not allow for 360.degree. rotation nor the low friction arrangement of a roller bearing mast support.
Accordingly, it is an object of the present invention to provide a jib crane configuration in which the horizontal boom may be easily adjusted to a level condition.
It is another object of the present invention to provide a 360.degree. rotation jib crane for long-length booms equipped with a turnbuckle cable boom support.